The present invention relates generally to host/slave communications and more particularly to an independently powered current probe system coupled to a measurement test instrument via a communications bus.
Current probes measure the flux field generated by the movement of electrons through a conductor. The flux field surrounding the conductor is converted to a linear voltage output that can be displayed and analyzed on a measurement test instrument, such as an oscilloscope. One type of current probe is an AC only probe. AC only probes are configured with either a solid core or a split core and are passive devices that do not require external power. AC/DC current probes generally have a split core configuration and include a Hall Effect device for producing a voltage output in response to a DC generated flux field. AC/DC current probes require a voltage source to provide power for biasing the Hall Effect device and for generating a bucking current through the coils of the current probe. An example of an AC/DC current probe system is the A6312 current probe powered by an AM503B Programmable Current Probe Amplifier and TM500 Power Module manufactured and sold by Tektronix Inc., Beaverton Oreg. The TM500 Power Module provides electrical power to the AM503B Programmable Current Probe Amplifier.
The current probe amplifier has two front panel connectors with one connector for coupling the current probe to the amplifier and the other connector for coupling the signal output of the current probe amplifier to the measurement instrument, such as an oscilloscope. The signal output connector is a BNC type connector that accepts a mating BNC type connector affixed to a coaxial cable. The other end of the coaxial cable has a BNC type connector that mates with a BNC type connector mounted on the front panel of the oscilloscope and coupled to an input signal channel of the oscilloscope. The only signal passing between the current probe amplifier and the oscilloscope is the signal output from the amplifier.
Tektronix, Inc. introduced a probe connection system for voltage measurement probes that provides voltage power and communications between the probe and the oscilloscope through a probe/oscilloscope interface connector. The interface connector includes a BNC type connector for coupling a signal under test from the probe tip to the oscilloscope. Surrounding the conventional BNC type connector on the oscilloscope are connector landing pads for coupling voltage power, offset voltage and serial data communications lines to the probe. The preferred communications is via an I2C type communications bus. The connector landing pads are connected to the probe interface by means of spring-loaded pins on the probe connector. The electrical and mechanical characteristics of the above probe connection system is described in U.S. Pat. No. 4,672,306 to Thong and U.S. Pat. No. 4,708,661 to Morland et al. and are said to exhibit Level 2 capability. U.S. Pat. No. 6,232,764 to Rettig et al. and U.S. Pat. No. 6,385,550 to Jansen et al. describe accessories, such as probes, with internal adjustments controlled by a host. These additional capabilities are implemented through the Level 2 interface and are said to exhibit Level 3 capabilities. The voltage measurement probes exhibiting Level 2 and Level 3 capabilities include a Level 2 or Level 3 coding resistor coupled to the communications bus data line and ground. The data and clock lines generally have pull-up resistors in the measurement instrument. Connecting the voltage probe to the measurement instrument produces a voltage divider network that signals the measurement instrument that a Level 2 probe has been connected. The oscilloscopes responds by reading parameter data from an EEPROM in the probe. Disconnecting the probe from the measurement instrument allows the data line to float high signaling the measurement instrument that a probe is not connected to the measurement instrument. In addition, disconnecting the voltage probe from the instrument removes the voltages for the probe.
The P6248 Differential Probe, manufactured and sold by Tektronix, Inc., provides variable attenuation levels of 1× and 10× and has Level 2 capabilities. The attenuation levels are controlled by a mechanical switch mounted on the compensation box of the probe. When the P6248 is connected to a Level 2 compatible oscilloscope, the data line is pulled down by the Level 2 coding resistor indicating to the oscilloscope that a Level 2 probe has been connected and parameter data stored in the EEPROM needs to be read. Switching the attenuator setting from one setting to the next, causes the coding resistor to be momentarily disconnected from the data line which allows the line to float high. At the same time a different EEPROM is connected to the data line matching the new attenuator setting. When the attenuator switch is completely toggled to the new setting, the coding resistor is reconnected to the data line indicating the EEPROM needs to be read. At no time during the switching of the attenuator settings is the probe disconnected from the oscilloscope and at no time is power removed from the probe.
It is desirable to provide Level 2 type probe communications between an oscilloscope and an AC/DC current probe system. However, problems arise when the current probe system is connected to a Level 2 oscilloscope and either the current probe amplifier or the oscilloscope is powered down. The powered down device will essentially ground the I2C clock and data lines, disrupting I2C communications in the other device. If the oscilloscope is powered on and the current probe amplifier is off, the oscilloscopes I2C lines will be pulled down. This is detrimental to other probes or devices attached to the oscilloscope that may share the I2C lines. Since both the clock and data lines typically have pull-up resistors in the oscilloscope, this can also bring down the voltage levels in the oscilloscope coupled to the current probe amplifier. In the other case, when the current probe amplifier is powered on and the oscilloscope is off, the current probe amplifier will be affected. Both the clock and data lines will appear to be pulled to ground by the oscilloscope, falsely indicating I2C communications. This will lock-up control functions on the front panel of the current probe amplifier. Also, any internal communications between the current probe amplifier internal microcontroller and EEPROM devices in the current probe amplifier will be impossible.
What is needed is a communications bus management circuit for a current probe system that controls the communications bus connectivity between the measurement test instrument and current probe amplifier. The communications bus management circuitry needs to detect the presence of a connected and powered measurement test instrument to the current probe amplifier. The communications bus management circuitry further needs to detect the presence and absence of the current probe connected to the current probe amplifier and provide communications to the measurement test instrument when a current probe is connected or changed on the current probe amplifier.